To meet the continued fast growing demand of mobile data, the wireless industry needs solutions that can achieve high order of spatial re-use of limited spectral resources. One method for increasing spatial re-use of wireless spectrum is MIMO, especially large scale Multi-User MIMO (MU-MIMO), often referred to as massive MIMO. In a wireless communication system, a wireless node with multiple antennas, a Base Station (BS) or a User Equipment (UE), can use beamforming in downlink (DL) or uplink (UL) to increase the Signal-to-Noise Ratio (SNR) or Signal-to-Interference-plus-Noise Ratio (SINR), hence the data rate, of the links with other wireless nodes. A BS with MU-MIMO can beamform to multiple UEs simultaneously in a frequency and time block, e.g., a Resource Block (RB), i.e., using spatial multiplexing to provide capacity growth without the need of increasing the bandwidth. In a large-scale MIMO or massive MIMO system, a BS may be equipped with many tens to hundreds or even thousands of transmit (Tx) chains and receive (Rx) chains and simultaneously beamform to tens of UEs using the same RB. Let the number of BS Tx and Rx chains be M and the number of simultaneously served UEs on the RB be K. There is a need of a hardware RF channel emulator capable of emulate in real-time M×K RF channels. In a TDD system, it is desired that such an emulator maintains the reciprocity of the over the air channel (OTA) so that the BS can estimate the DL OTA channel using UL signaling. This allows the channel estimation in a massive MIMO system to scale with K rather than M, thus significantly reduces the overhead of channel estimation in a massive MIMO system. Furthermore, the antennas in a massive MIMO system may be integrated with the RF circuits and there are no antenna ports for using cables to connect the BS RF paths to the channel emulator. Even if the BS has antenna ports, it will be a hassle to connect many tens or even more RF ports using cables. There are no prior art hardware RF channel emulator exits that meet these needs. This invention presents circuits and methods for building a hardware RF channel emulator that meet the needs of testing of massive MIMO systems.